The present invention generally relates to light-emitting diodes (LEDs), and more particularly, to the design of a lighting apparatus and lighting system using such in a manner that maximizes the benefits of LEDs to satisfy difficult lighting requirements.
By now it is well known that the use of LEDs in general lighting applications yields substantial benefits: long operating life, high efficacy, and precise control of light are at the forefront. However, it is also well known that to get the most out of LEDs a number of factors must be considered: temperature (both ambient and junction) and luminaire design, for example. LEDs are quickly becoming the light source of choice for architectural or aesthetic lighting applications (e.g., façade lighting, holiday lighting, indoor track lighting, etc.), but their usefulness in long-term, large-scale lighting applications has been more slowly realized. This is due, at least in part, to the tremendous efforts needed to control such things as ambient and junction temperature, as well as the efficiency of the luminaire design. In essence, because the benefits of operating LEDs are so closely coupled to the particulars of the lighting application, there is no such thing as a standard large-scale LED lighting fixture. Couple this with only a rudimentary understanding the industry has of how long LEDs can be operated effectively, and it can be seen that there is significant room for improvement in the art.
Consider an outdoor bridge spanning some length and accommodating some number of lanes of traffic in both directions; assume this bridge is used heavily both day and night. For the safety of nighttime drivers, the road on the bridge must be illuminated; here lies an application that exemplifies the challenges faced by today's lighting designers. Cost effectiveness suggests lighting fixtures should be affixed to existing structural features (e.g., to avoid the cost of support structures and the cost to shut down multiple lanes of traffic to erect said structures); however, mounting height and aiming of said fixtures must be considered so not to cause glare or create other adverse driving conditions (the difficulty of which is exacerbated because traffic flows in both directions). The lighting designer must take into account placement of the fixtures, weight of the fixtures, and outward design of the fixtures to ensure both adequate distribution of light on and about the target area, and distribution of stresses on the poles (e.g., because of wind loading). At all times, there are competing design considerations. For example, LEDs offer the benefit of long life (a boon to cost effectiveness), but must be used in great quantity to produce the light needed (a detriment to cost effectiveness). A plurality of light sources means the composite light projected therefrom can be precisely controlled to suit the target area, but it also means additional optical elements for each light source (adding to the cost and weight of each fixture).
Additionally, there is a vested interest in designing the lighting system at the onset for long-term use; in the aforementioned example, it is simply not economically feasible to shut down multiple lanes of traffic over the life of the system to perform maintenance, re-lamp, etc. Thus, LEDs are a natural choice; their long life removes some concerns with long-term maintenance. However, because LEDs have such a long life they have not been fully tested; thus, there are no definitive answers as to how long LEDs can operate and how severely the light output will degrade over time due to thermal losses and lumen depreciation (not to mention initial efficiency losses due to driver inefficiencies and luminaire design). The Illuminating Engineering Society of North America (IESNA) has recently recommended standards for testing LEDs (see IES LM-79) and measuring lumen depreciation (see IES LM-80), but the scope is limited and does not define or provide estimations for the lifespan of LEDs.
The art is at a loss; in the time it would take to fully test an LED, the technology will have advanced and the data will not be particularly useful. In the meantime, there are lighting applications that may benefit from the long life of LEDs provided that long life can be assured. What is needed are means for reasonably assuring the long life of LEDs in a manner that is reliable and, unlike current maintenance strategies, cost-effective for applications like the aforementioned bridge. Further, what is needed are means for reasonably assuring an acceptable light level over said life; there is little benefit to maintaining an LED lighting system long-term if the light is allowed to degrade to the point of uselessness. Still further, what is needed is a standardized approach to developing large-scale LED fixtures—particularly ones for outdoor use—that can be used with said means for assuring the long life of LEDs so to address current needs. Thus, there is room for improvement in the art.